The present invention relates to an objective lens for an optical pick-up that is capable of using a plurality of kinds of optical discs whose recording densities are different from each other.
In general, diameter of a beam spot formed on a recording layer of an optical disc is closely related to the recording density of the optical disc. That is, the spot diameter is required to have a suitable size for appropriately covering a track on the recording layer in width. The disc with higher recording density has narrower track width, which requires a smaller spot size. On the contrary, the disc with lower recording density has wider track width, which requires a larger spot size.
When the spot diameter is much larger than the track width, reproduced signal may include jitter, which is undesirable effect of the reflected light from adjacent tracks on the reproduced signal. On the other hand, when the spot diameter is much smaller than the track width, particularly for an optical system that reproduces the recorded signal from an optical disc such as a compact disc (CD) through the use of diffraction of light, the system may miss reproducing the signal due to insufficient diffraction.
Since the spot diameter becomes smaller as a wavelength of light becomes shorter and as numerical aperture (NA) becomes larger, the optical system for a digital versatile disc (DVD) with high recording density requires a shorter wavelength and a higher NA, and the optical system for the CD with low recording density requires a longer wavelength and a lower NA. Further, the reflection characteristics of a CD-R (CD recordable) require the laser source whose emission wavelength is longer than 780 nm. Accordingly, the optical pick-up that is capable of using a DVD and a CD-R is needed to provide a light source that has a relatively short emission wavelength about 650 nm and a light source that has a relatively long emission wavelength about 780 nm.
A conventional optical system of the optical pick-up compatible between a CD and a DVD limits the aperture when a CD is used, thereby forming beam spots having suitable size for both a CD and a DVD. An adjustable aperture stop or a dichroic filter is located between the light source and the objective lens to limit the aperture.
However, when an aperture stop or a dichroic filter is installed as a separate element, the number of components is increased, resulting in a disadvantage in size and weight.